1. Field of the Invention
The present invention relates to a method for manufacturing a thin film transistor (TFT) formed by using a crystallized semiconductor for a semiconductor layer including a channel forming region, a source region and a source region, particularly, a method for manufacturing a semiconductor having a good crystal characteristic by laser beam radiation.
2. Description of the Related Art
An active matrix type of liquid crystal display device capable of highly fine display has been manufactured so popularly. In the active matrix type of liquid crystal display device, a TFT is provided in each pixel in a pixel portion as a switching element for driving a liquid crystal. The TFT provided in the pixel portion is switched between on and off to change an orientation of the liquid crystal in order to carry out display.
Especially in a crystallized semiconductor film having high electric field effect mobility (typically, a poly-silicon film) of all others, a carrier moves so fast that, when such crystallized semiconductor film is used for a semiconductor layer including the channel forming region, a source region and a source region, it is possible to provide on a substrate same as the pixel portion a drive circuit capable of corresponding to writing of image data even having high resolution, the drive circuit being required to operate at high speed. Such crystallized semiconductor is on the way to practical use.
Demand of a market, however, does nothing but increase for further fineness, higher brightness and lower cost of a liquid crystal display device. In order to solve a cost problem, it is necessary to develop a technology in which a changeover from a quartz plate, which is expensive per substrate, and thereby, raises a price of a liquid crystal display device as a final product, to a cheap plate (a glass plate, for example) and a changeover from a high temperature process, which needs a manufacturing cost such as electric power, to a low temperature process are possible.
Therefore, a laser beam radiation method or a crystallization method using a catalyst element is used as a method for providing a good element at a cheap price and obtaining a good semiconductor film.
Radiation of a semiconductor film with a laser beam enables the semiconductor film to be crystallized or improved in the crystal character. It is because the semiconductor film is fused due to energy of a laser beam and forms innumerable nucleuses so that respective nucleuses would grow mainly in a direction parallel to a film surface of the semiconductor film to form a crystal particle and be solidified.
In growth of such crystal particle after the laser beam radiation, formed on a semiconductor film a convex portion having the height almost equal to the thickness of the semiconductor film due to a collision between adjacent crystal particles.
In the case that a TFT is manufactured by using semiconductor film, which is obtained in the laser beam radiation process under such condition that a convex portion is formed on a surface thereof and on which the formed convex portion remains, as a semiconductor layer including a channel forming region, a source region and a source region, the roughness of the surface of the semiconductor film is reflected in a gate insulating film and a gate electrode, which are later formed on the semiconductor layer, so that it would cause a problem of dispersion of an element characteristic.
Further, there are problems that leakage easily occurs in an OFF operation of a TFT (a drain current flowing in the OFF operation of a TFT becomes high) and that electrostatic focusing occurs to raise an OFF current, since the film thickness of a semiconductor layer is thick at the convex portion. In addition to the above, the roughness of an interface between the semiconductor layer and the gate insulating film traps a carrier (electron) flowing through the channel forming region so that the carrier would become a fixed electric charge to vary a threshold voltage, which causes decline in reliability.
On the other hand, a semiconductor film having high electric field effect mobility can be obtained in the laser beam radiation process under such condition that a convex portion is formed as described above. There is, accordingly, an antinomy relation.
A purpose of the invention is, in order to practically manufacture a TFT having a good characteristic, to put into practice a method for leveling the surface of a semiconductor layer without increasing the number and the complication of manufacturing processes as well as without deteriorating the crystal characteristic and a method for leveling a surface of a semiconductor layer to stabilize an interface between the surface and a gate insulating film.
Another purpose of the invention is to put into practice a method for manufacturing a semiconductor device represented by a liquid crystal display device in which a TFT comprising such semiconductor layer is used for a circuit and/or a switching element.
Thus, the invention is a method for manufacturing a semiconductor device comprising steps of:
forming a semiconductor film over an insulating surface;
forming an oxide film on a surface of the semiconductor film; and
radiating the semiconductor film with a laser beam in a first condition, a second condition and a third condition in order in an atmosphere of one kind of gas or a mixed atmosphere of plural kinds of gas, the gas being selected from hydrogen and inert gas,
wherein the laser beam in the first condition is a laser beam having a first energy density, a first wavelength, and a first pulse width,
wherein the laser beam in the second condition is a laser beam having an energy density, a wavelength and a pulse width respectively lower than those of the laser beam in the first condition, and
wherein the laser beam in the third condition is under a condition that the energy density is higher than that of the first condition by 30 to 60 mJ/cm2.
The invention is also a method for manufacturing a semiconductor device including steps of:
forming a semiconductor film over an insulating surface;
forming an oxide film on a surface of the semiconductor film; and
radiating the semiconductor film with a laser beam in a first condition, a second condition and a third condition in order in an atmosphere of one kind of gas or a mixed atmosphere of plural kinds of gas, the gas being selected from hydrogen and inert gas,
wherein the laser beam in the first condition has a first energy density, a first wavelength and a first pulse width, and radiation of the laser beam in the first condition forms a crystallized semiconductor film to crystallize the semiconductor film,
wherein the laser beam in the second condition has an energy density, a wavelength and a pulse width lower than those of the laser beam in the first condition, and radiation of the laser beam in the second condition eliminates the oxide film, and
wherein the laser beam in the third condition is a laser beam whose energy density is higher than that of the first condition by 30 to 60 mJ/cm2, and radiation of the laser beam in the third condition levels a surface of the crystallized semiconductor film.
Further, the invention is a method for manufacturing a semiconductor device including steps of:
forming a semiconductor film over an insulating surface;
forming an oxide film on a surface of the above semiconductor film; and
radiating the semiconductor film with a laser beam in a first condition, a second condition and a third condition in order in an atmosphere of one kind of gas or a mixed atmosphere of plural kinds of gas, the gas being selected from hydrogen and inert gas,
wherein the laser beam in the first condition has a first energy density, a first wavelength and a first pulse width, and radiation of the laser beam in the first condition forms a crystallized semiconductor film to crystallize the semiconductor film,
wherein the laser beam in the second condition has an energy density, a wavelength and a pulse width lower than those of the laser beam in the first condition, and radiation of the above laser beam in the second condition eliminates the oxide film, and
wherein the laser beam in the third condition is a laser beam whose energy density is higher than that of the first condition by 30 to 60 mJ/cm2, and radiation of the laser beam in the third condition makes a difference between top and bottom points of a surface of the crystallized semiconductor film 6 nm or less.
The invention is further a method for manufacturing a semiconductor device including steps of:
forming a semiconductor film over an insulating surface;
forming a crystallized semiconductor film to crystallize the semiconductor film;
forming an oxide film on a surface of the crystallized semiconductor film; and
radiating the crystallized semiconductor film with a laser beam in a first condition, a second condition and a third condition in order in an atmosphere of one kind of gas or a mixed atmosphere of plural kinds of gas, the gas being selected from hydrogen and inert gas,
wherein the laser beam in the first condition has a first energy density, a first wavelength and a first pulse width, and radiation of the laser beam in the first condition crystallizes the semiconductor film to form the crystallized semiconductor film,
wherein the laser beam in the second condition has an energy density, a wavelength and a pulse width lower than those of the laser beam in the first condition, and radiation of the laser beam in the second condition eliminates the oxide film, and
wherein the laser beam in the third condition is a laser beam whose energy density is higher than that of the first condition by 30 to 60 mJ/cm2, and radiation of the laser beam in the third condition levels a surface of the crystallized semiconductor film.
In the invention, the oxide film is formed by contacting the surface of the semiconductor film with a solution or gas containing ozone.
The invention is also a method for manufacturing a semiconductor device including steps of:
forming a semiconductor film over an insulating surface;
adding a catalyst element to the semiconductor film so as to form a crystallized semiconductor film by heat treatment;
forming an oxide film on a surface of the crystallized semiconductor film;
radiating the crystallized semiconductor film with a laser beam; and
carrying out a heating process in order to transport the catalyst element included in the crystallized semiconductor film to a gettering site,
wherein the step of radiating the crystallized semiconductor film with a laser beam, radiation with a laser beam in a first condition, a second condition and a third condition is carried out in order in an atmosphere of one kind of gas or a mixed atmosphere of plural kinds of gas, the gas being selected from hydrogen and inert gas,
wherein the laser beam in the first condition has a first energy density, a first wavelength and a first pulse width, and radiation of the laser beam in the first condition improves a crystal characteristic of the crystallized semiconductor film,
wherein the laser beam in the second condition has a energy density, a wavelength and a pulse width lower than those of the laser beam in the first condition, and radiation of the laser beam in the second condition eliminates the oxide film, and
wherein the laser beam in the third condition is a laser beam whose energy density is higher than that of the first condition by 30 to 60 mJ/cm2, and radiation of the laser beam in the third condition levels a surface of the crystallized semiconductor film.
In the invention, the inert gas is selected from the group consisting of nitrogen, argon, helium, neon, krypton and xenon.
Furthermore, in the invention, the catalyst element is one kind of or plural kinds of element selected from the group consisting of Fe, Ni, Co, Ru, Rh, Pd, Os, Ir, Pt, Cu and Au.
Furthermore, in the invention, the heat treatment is a heating process in which heating wire, lamp light source or heated gas is used as a heat source.
Furthermore, in the invention, the gettering site includes one kind of or plural kinds of element selected from the group consisting of He, Ne, Ar, Kr and Xe.
Furthermore, in the invention, the gettering site includes one kind of or plural kinds of element selected from elements belonging to the 15th group in a periodic table.
Furthermore, in the invention, the energy density in the first condition is 300 to 500 mJ/cm2.
A semiconductor film can be crystallized by radiation with the laser beam in the first condition in an atmosphere of one kind of gas or a mixed atmosphere of plural kinds of gas, the gas being selected from nitrogen, hydrogen or inert gas. The surface of the crystallized semiconductor film obtained by the first condition laser beam radiation has a convex portion.
Following to the above, an area radiated with the laser beam in the first condition is radiated with a laser beam in the second condition in a process room having the same atmosphere. Thus, an oxide layer is formed on a semiconductor film before the first condition laser beam radiation can be eliminated.
Following to the above, an area, which is radiated with the laser beam in the second condition and in which an oxide layer is eliminated, is radiated with a laser beam in the third condition in a process room having the same atmosphere. Thus, the surface of the crystallized semiconductor film can be leveled.
According to the invention, the second condition laser beam radiation is carried out for an area radiated with the first condition laser beam in a process room having a same atmosphere after the first condition laser beam radiation is carried out for crystallizing a semiconductor film or improving the crystal characteristic, and thereby, an oxide film can be eliminated. The surface of the crystallized semiconductor film can be also leveled by, following to the above, the third condition laser beam radiation of an area, which is radiated by the laser beam in the second condition and in which an oxide layer is eliminated, in a process room having a same atmosphere. The laser beam radiation of a semiconductor film in order from the first condition, the second condition and the third condition enables a process from crystallization to leveling to be performed without changing an atmosphere in a process room, which can shorten time for operation.
Moreover, a process substrate is not contaminated since the process from crystallization to leveling can be continuously carried out without exposure to the air.